The present invention relates to a process for the enhancement of the cycle life of zinc chromium based catalyst in the synthesis of 2-methylpyrazine. More particularly, the present invention relates to an improved process for the enhancement of the cycle-life of the catalyst used in the synthesis of 2-methylpyrazine.
2-methylpyrazine is used as an intermediate in the production of pyrazinamide, an anti-tubercular drug. It is known to react ethylenediamine and propyleneglycol on a zinc-based catalyst by a vapour phase process to produce 2-methylpyrazine. 2-methylpyrazine, in turn, is converted to 2-cyanopyrazine by catalytic ammoxidation of 2-methylpyrazine using ammonia and oxygen or air. In the last step, 2-cyanopyrazine is hydrolyzed to 2-amidopyrazine, which is popularly called as pyrazinamide.
The duration of time when the catalyst exhibits continuous steady-state activity/selectivity before it starts showing unacceptable activity and selectivity and requires regeneration is normally defined as its cycle-life. After regeneration, the catalyst regains its activity/selectivity either fully or partially and is used for the next cycle. After a few regenerations, the catalyst gets deactivated by exhibiting unacceptable activity/selectivity or both even after regeneration and needs to be replaced with a fresh catalyst. The total time for which the catalyst is put into operation before discharge is called the total life or simply the life of the catalyst. Longer cycle-life and thus longer catalyst life are preferred for catalysts operating in fixed bed reactors. The commercial process becomes economical when the catalyst not only offers high activity and high selectivity towards the required product but also possesses sufficient cycle-life and thus, the total life.
Reference is made to the following zinc-based catalysts which were reported in the prior art for the preparation of 2-methylpyrazine wherein (i) Zinc catalyst promoted with one or more of cobalt, nickel, iron, chromium and aluminum (British Patent 1565117). (ii) ZnCr2O4-incorporated ZnO with Pd as promoter (Journal of Applied Catalysis Vol. 29, 1987; Journal of Catalysis, Vol. 111, 1988 and Journal of Chemical Society Transactions Vol. 84(7), 1988; Indian Patent Application No. 280/DEL/92 and Indian Patent Application No. 281/DEL/92).(iii) Catalyst with zinc and chromium in a molar ratio of 1:3 (Journal of Catalysis, Vol.111, 1988; Catalysis, Vol.3, Rheinhold Publications, New York, Page 349, 1955; Catalytic Manufacture, Dekker, N.Y., 1983) (iv) Zinc-chromium based catalyst with non-metallic sulphate or bisulphate as promoter (Indian Patent No.176919, 1993)
The drawbacks of the prior art are as follows: The catalyst referred to above in item (i) has a poor cycle life, though the 2-methylpyrazine yields are promising. Zinc based catalysts promoted with Ce, Mg etc., though offer high initial activity, their life times are of the order of 25 hours. Catalyst reported by Forni et al, cited in item (ii) above, has the first cycle life of about 150 hours. Subsequent second and third cycle lives after regenerations were lower compared to the first cycle. The total catalyst life after three regenerations (with the catalyst giving a maximum of 50% conversion), was reported to be 450 hours. Non-metallic sulphate or bisulphate promoted zinc-based catalysts reported in item (iv) referred above also were reported to be tested for 144 hours with acceptable conversions and yields. Thus, the catalysts reported in the prior art possess their cycle lives less than 150 hours.
Accordingly the main object of the present invention is to provide a process for the enhancement of the cycle-life of the catalyst used in the synthesis of 2-methylpyrazine from ethylenediamine and propyleneglycol by a vapour phase catalytic process which obviates the drawbacks as detailed above.
Another object of the present invention is to provide the operating conditions wherein the conversions with respect to ethylenediamine and propyleneglycol are in the order of 100%.
Yet another object of this invention is to provide the operating conditions wherein the yield of 2-methylpyrazine is obtained in the range of 70-80%.
Yet another object of the present invention is to provide the operating conditions wherein the product of the reaction contains pyrazine as the main byproduct and its composition in the product is less than 10-12%.
Still another object of this invention is to provide a method for the regeneration of the catalyst after first cycle life such that the regenerated catalyst again gives ethylenediamine and propyleneglycol conversions and 2-methylpyrazine and pyrazine selectivities similar to those obtained on the fresh catalyst.